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Yes single loop field with two units. Usually we use an 18 or 25K BTU w2w geo unit for domestic so as long as your loop isn't to big you should be fine adding the dedicated hot water. If you were to do two 6 ton units then your loop may have to much head pressure to be able to be properly flushed. If you think about it the average electric water heater is 4500 watts X 3.413 BTU's per watt is 15,358 BTU's of the average electric water heater so using a 18K BTU dedicated domestic water unit should be about equivalent or using the 25K BTU unit will do more than enough water even at low EWT's, plus the other benefit of dedicated hot water is that while the house is rejecting heat during cooling the domestic unit will be taking the heat from the loop so they work in combination.

So as I'm continuing to research this I discovered that water to water units aren't 2-stage which sort of makes my ideas for how to apply a single larger unit for all my purposes a bit harder or just completely impractical. I would have to use a second buffer tank for chilled water or the cooling side would be way oversized and then I wonder how the unit would perform with regard to dehumidification. How does one even achieve staging with a chilled water unit?
So, I think I'm going to be looking at what Sky had recommended in his first post in this thread, which is a split 2-stage air unit wiith a dedicated (optional) water 2 water for the radiant and domestic production. Or, the waterfurnace Synergy 3D still appears to meet the needs as well - I'll have to get more information on that too.

Some brands have multi stage water to water units, Waterfurnace does but only in the 8 ton and larger size. The chilled water will not dehumidify as well as a refrigerant coil because the water will not get to as low of a temperature as refrigerant. I know a lot of people in my area do the reversible water to water units because they can be less costly than an all in one but you also lose performance IMO.

your cooling side wouldn't be oversized because your cooling the water in the buffer tank and pulling out of that to your properly sized hydronic coil on furnace. If you do 2 heat pumps use 1 well field B&D Qt flowcenter.

If I'm thinking of this the right way, JCT, I would need to have two buffer tanks for conditioned water - one for the hydronic side that would stay hot year round that could serve the floor and domestic water needs, and another that would serve the forced air side and would be heated in the winter and cooled in the summer. Add to those a buffer tank for the desuperheater to temper and a final domestic hot water tank and I've got 4 tanks standing around.
What do you guys think of this - if I end up using either a Synergy3D or a dedicated water to water unit to heat both for radiant floors and domestic hot water, am I really gaining anything using the desuperheater? In the winter, I am simply stealing some of the units heating capacity to temper hot water, and in the summer I am grabbing the heat before it makes it to the loop field, but if I ultimately get the heat from the field to put into the hot water, do I really need a buffer tank and the desuperheater? That would at least eliminate one of the tanks needed with a single Water-to-water geo unit.
JCT - what temperature are you trying to keep the cooling water in a buffer tank to be used for cooling? 50*? 40*?
Thanks guys - very helpful to be able to think out loud about this.

You only need 1 buffer tank for your heating and cooling, all your radiant and hydronic coil will pump out of that 1 tank it will be either a hot tank or a cold tank depending on the season. you can't run your domestic and radiant together. Your desuperheater can get piped into your water heater or you can get another water heater and desuper into that and from that to you water heater
check this outhttp://www.bdmfginc.com/BDPdfs/HSS.pdf

You only need 1 buffer tank for your heating and cooling, all your radiant and hydronic coil will pump out of that 1 tank it will be either a hot tank or a cold tank depending on the season. you can't run your domestic and radiant together. Your desuperheater can get piped into your water heater or you can get another water heater and desuper into that and from that to you water heater
check this outhttp://www.bdmfginc.com/BDPdfs/HSS.pdf

I don't quite understand this wording on page 2.
"(Tank is not designed to be hot/chilled water storage device)" yet the next line says "Hydronic pump relay standard sensor for one stage heat, two stage heat or heating and cooling built in."

How can it not be a chilled water storage device yet have relays for cooling built in?

That is a good question typo or maybe they are saying it has to be hot or cold not both at the same time? They are for both though. I have put about 100 of these in and they work great along with their ground loop pump pack

I've used the QT non-pressurized flow center for all my installs and love it - I'm going to be trying out their prefabbed pvc manifold with the mechanical fittings for my next one - should save a bit of time making it on the jobsite. Definitely going to consider using that HSS Buffer tank for one of the options on this job. Thanks guys.

Question for you guys. I would like to offer the HO the option of generating all their Domestic hot water via Geothermal (with some kind of electric or propane backup). They currently have a 75 gallon natural draft propane water heater. It's rated for 75,000BTUhr and 130 gallons first hour rating. I somewhat doubt they require that amount of hot water at any given time, but I also don't want to be the guy that undersizes it and they hate their system in the future.

I have two trains of thought here - provide a reasonable amount of preheated hot water in a storage tank - at least 50 gallons. Use the geo desuperheater with a buffer tank so I'm not trying to heat 50 degree well water, but hopefully 90+ degree buffer water.

Beyond that, how much capacity should I be adding or what size dedicated W2W should I be looking at to meet this need? The smaller WF w2w units are only 1.5 or 2 tons that seem to be for just domestic production.

Should I just use a larger storage tank (75-100 gallons?) and let the Geo do its thing over time? The other thought is to have my auxilliary be something like a boiler/domestic combo unit made by navien or Triangle tube that could make up for any need instantly - but that's an awefully expensive backup solution. Looking for some guidance.

I would put in at least an 80 gallon storage tank with an NSW040 dedicated water unit and then run a desuperheater off the other unit to a 50 gallon preheat tank. That should give you enough heat for most applications unless they are filling up 100+ gallon jacuzzi tubs. I have noticed a lot of houses on wells have 80 gallon tanks. This could be because the well can't keep up with a 50 gallon so they use the 80 so there is no buffer tank from the well. So the current 75K btu unit may not be needed at all. I would just ask about their water usage and take look at their tubs/showers

Here's a question I should probably already know the answer to - if you use a dedicated water-to-water unit for the domestic hot water, do you need some kind of heat exchanger between the domestic water and the unit, or can you run the domestic directly throught the unit (like the desuperheater)? I know the desuperheaters on the units I've used have double wall vented heat exchangers so they're safe for potable water, but what about the primary on a W2W unit?